Process for the production of acetic acid C4-esters

ABSTRACT

Process for treating a mixed C 4  stream comprising iso-butene and 1,3-butadiene by (a) reacting the stream with acetic acid in an addition reactor, (b) withdrawing product comprising iso-butene, sec-butenyl acetate, n-butenyl acetate and t-butyl acetate, and recovering n-butenyl acetate from the product and separating and recycling t-butyl acetate to the addition reactor. The process provides increased efficiency of acetic acid utilisation together with useful products, e.g. n-butenyl and sec-butenyl acetates which can be separated and sold as such or converted to other products. The unreacted isobutene can be separated and used as such, or recycled and allowed to dimerise to di-isobutene, another useful product.

[0001] This invention relates to a process for treating a mixed C₄stream comprising iso-butene and 1,3-butadiene.

[0002] In naphtha steam-cracking, naphtha is decomposed in the presenceof a steam diluent at a temperature between 700 and 900° C. The processproduces a number of products, including isomeric butanes, 1,3-butadieneand isomeric butenes (e.g. 1-butene, 2-butene and iso-butene). Thesecompounds may be separated as a mixed C₄ stream.

[0003] As described in WO 00/26175, mixed C₄ streams may be contactedwith acetic acid in the presence of a catalyst. Under the reactionconditions, butadiene in the C₄ stream reacts with acetic acid toproduce n-butenyl and sec-butenyl acetate. The sec-isomer may berecycled to the reactor. The n-isomer, on the other hand, is recoveredand hydrogenated to produce n-butyl acetate which is a useful solvent.

[0004] Not all the acetic acid initially fed to the reactor is consumedin the butadiene/acetic acid addition reaction. Instead, some of theacetic acid reacts with the iso-butene present in the mixed C₄ feedstockto produce t-butyl acetate. This by-product is isolated from the productmixture and cracked back to iso-butene and acetic acid. The iso-buteneis recovered by distillation, and sold, for example, as a feedstock forthe production of polyisobutene (PIB). The acetic acid is recycled tothe reactor, and may be re-consumed in one of the addition reactionsoccurring therein.

[0005] The cracking and distillation equipment required in the processof WO 00/26175 can add cost and complexity to the overall process. It istherefore among the objects of the present invention to provide analternative process for treating such mixed C₄ streams.

[0006] According to the present invention, there is provided a processfor treating a mixed C₄ stream comprising iso-butene and 1,3-butadiene,said process comprising:

[0007] a) reacting acetic acid with said stream in an addition reactor,

[0008] b) withdrawing from the addition reactor a product streamcomprising iso-butene, sec-butenyl acetate, n-butenyl acetate andt-butyl acetate, and

[0009] c) recovering n-butenyl acetate from the product stream,characterised in that

[0010] d) t-butyl acetate is recycled to said addition reactor.

[0011] For the avoidance of doubt, sec-butenyl acetate and n-butenylacetate have the following structures:

[0012] sec-butenyl acetate

[0013] n-butenyl acetate

[0014] Under the operating conditions of the addition reactor, isobuteneand acetic acid react together reversibly to form t-butyl acetate andhence under stable operating conditions isobutene, acetic acid andtertiarybutyl acetate are substantially in equilibrium. Thus, byrecycling the t-butyl acetate back to the reactor, the amount of t-butylacetate in the reaction loop eventually approaches a substantiallyconstant value. By controlling the amount of t-butyl acetate produced inthis manner, the amount of acetic acid consumed in the reaction betweeniso-butene and acetic acid is maintained at a substantially constantlevel. This leaves a significant proportion of the initial acetic acidfeedstock available for desirable reactions, such as the additionreaction with 1,3-butadiene. Although in the present inventioniso-butene is preferably not produced by cracking t-butyl acetate asdescribed in WO 00/26175, any unreacted iso-butene present in theoriginal mixed C₄ stream may be recovered, for example, by distillationor flash separation techniques.

[0015] The mixed C₄ stream employed as a feedstock in the presentprocess can be, for example, a by-product of a reaction, such as thedehydrogenation of butane or butene, or the steam cracking of naphtha.Such mixed C₄ streams frequently comprise isobutene and 1,3-butadiene.The mixed C₄ stream may also comprise one or more of isobutane,n-butane, 1-butene, trans-2-butene, cis-2-butene, 1,2-butadiene,propadiene, methyl acetylene, ethyl acetylene, dimethyl acetylene, vinylacetylene, diacetylene, and C₅ acetylenes. In one embodiment of thepresent invention, the mixed C₄ stream is a by-product of naphtha steamcracking comprising isobutane (e.g. 1-2% v/v), n-butane (e.g. 2-4% v/v),isobutene (e.g. 25-29% v/v), 1-butene (e.g. 8-10% v/v), trans-2-butene(e.g. 6-8% v/v), cis-2-butene (e.g. 3-5% v/v), 1,3-butadiene (e.g.43-48% v/v), 1,2-butadiene (e.g. 0-2% v/v), propadiene (e.g. 0-1% v/v),methyl acetylene (e.g. 0-1% v/v), ethyl acetylene (e.g. 0-1% v/v)dimethyl acetylene (e.g. 0-1% v/v), vinyl acetylene (e.g. 0-1% v/v),diacetylene (e.g. 0-trace) and C₅ acetylenes (e.g. 0-trace). It shouldbe understood that the precise composition of the latter stream may varydepending on factors such as the naphtha feed composition and the howthe cracker is operated.

[0016] As described in step a), the mixed C₄ stream is reacted withacetic acid in an addition reactor. The reaction conditions employed inthe addition step are described in detail in WO 00/26175. The relativemole ratios of butadiene in the mixed C₄ stream to acetic acidmaybe 5:1to 1:50, preferably, 1:1 to 1:10.

[0017] The addition reaction may be carried out at a temperature of 20to 140° C., preferably, of 40 to 90° C. The reaction may be carried outusing a homogeneous or heterogeneous catalyst, the latter beingpreferred. Suitable catalysts include acidic catalysts such as zeolites,and strong acid ion-exchange resins. Suitable ion-exchange resinsinclude Amberlyst 15® and Amberlite IR120®. A proportion of the acidicsites of such resins may be exchanged with bulky counterions such asalkyl pyridinium, quaternary alkyl ammonium, quaternary arsonium andquaternary phosphonium compounds. Tetra-phenylphosphonium counterions,for example, may be employed. These counterions may account for up to10% of the available acidic sites.

[0018] Water may be present in the addition step in an amount between0.05 and 50% w/w, preferably, 0.5 and 20w/w %, based on the total chargeto the reactor.

[0019] In certain cases, the activity of heterogeneous catalysts maydecrease after prolonged use. This may be due to blockage of activesites by 1,3-butadiene oligo- and polymerisation products. In suchcases, it may be advantageous to carry out the addition reaction underconditions of high shear, as high shear rates are believed to reduceblockage of active sites by the formation of such oligo- andpolymerisation products. Alternatively or additionally, polyrmerisationinhibitors may be added to the reaction mixture. Such inhibitors arewell known in the art. Where oligo- and polymerisation products arepresent in the product stream, however, these may be recovered andrecycled to the reactor.

[0020] The addition reaction may be carried out using any suitablereactor. For example, a fixed bed, slurry, trickle bed, bus loop, orfluidised bed reactor may be employed.

[0021] The reaction between the mixed C₄ stream and acetic acid producesa product stream, which is withdrawn from the addition reactor in stepb). This product stream comprises addition products including, interalia, n-butenyl acetate, sec-butenyl acetate and t-butyl acetate.Preferably, such addition products account for 1 to 99% w/w, forexample, 5 to 50% w/w of the product stream. The n-butenyl andsec-butenyl acetates result from the addition of acetic acid tobutadiene, whilst the t-butyl acetate results from the reaction betweenacetic acid and iso-butene. Such addition reactions, however, do notgenerally go to completion and are controlled by a number of factorsincluding how the reaction is conducted (e.g. LHSV), reaction kineticsand equilibrium constants. For this reason, unreacted isobutene and,optionally, unreacted 1,3-butadiene are also present in the productstream. These unreacted C₄ components are relatively volatile, and maybe separated, for example, by gas disengagement using any suitableseparation unit, such as a flash drum. During such a separation step,other volatile C₄ components in the product stream, such as unreactedisomeric butanes, 1-butene and 2-butene may also be separated. Wherebutadiene is present in the separated mixture, the separated mixture maybe selectively hydrogenated. This selective hydrogenation steppredominantly converts butadiene to 1-butene. Additionally, someisomerisation to 2-butene can occur, as well as further hydrogenation tobutane.

[0022] The separated mixture of unreacted C₄ components may be used as afeedstock, for example, for alkylation, or for steam cracking.Alternatively, the mixture of unreacted C₄ components may be separated(e.g. by physical and/or chemical methods) into one or more componentsfor sale or use. Isobutene, for example, may be recovered andpolymerised to produce polyisobutene (PIB). 1-Butene and/or 2-butene maybe separated, for example, as a mixture and used as a fuel additive.

[0023] In addition to the compounds mentioned above, the product streammay also comprise polymerisation by-products such as C₈ olefins (e.g.di-isobutene from isobutene) octatrienes (e.g. from butadiene+butadiene)and octadienes (e.g. from butadiene and isobutene ), C₁₂ olefins (e.g.from vinyl cyclohexene+butadiene, or C₈ olefin+butadiene), C₈ acetates,C₁₂ acetates and higher oligomeric materials. It may be desirable toremove one or more of such components from the product stream, forexample, by distillation. Di-isobutene recovered from the product streamcan, if desired, be converted by hydrogenation to iso-octane, a valuablefuel additive.

[0024] In step c), n-butenyl acetate is recovered from the productstream. This may be carried out using any suitable separating unit, forexample, one or more distillation columns. An example of a suitableapparatus for performing such separation steps is described in WO00/26175, particularly with reference to FIG. 2 of that document. Thefull disclosure of WO 00/26175 is incorporated herein by reference. Oncerecovered, the n-butenyl acetate may be, if desired, cracked back tobutadiene and acetic acid, or recycled to the reactor. Where then-butenyl acetate is cracked back to butadiene and acetic acid, at leastone of these components may be recycled to the reactor.

[0025] Preferably the recovered n-butenyl acetate is hydrogenated toproduce butyl acetate. The hydrogenation may be, for example, carriedout under heterogeneous conditions over any suitable catalyst. Examplesof suitable catalysts include ruthenium, platinum, nickel (e.g. Raneynickel) and palladium. These metals may be employed as elemental metalsor metal compounds. Although unsupported catalysts may be employed, itis preferable to use catalysts supported on inert carriers, such ascarbon or siliceous supports. Preferred catalysts include supportedRaney nickels, and ruthenium on carbon.

[0026] The hydrogenation may, for example, be carried out at 20 to 250°C., preferably, 40 to 200° C. The hydrogenation maybe, for example,carried at a pressure of 1 to 100 barg, preferably, 5 to 50 barg. Thehydrogenation can be carried out, for example, in slurry and/or flowreactors.

[0027] As described in step d), t-butyl acetate is recovered from theproduct stream and recycled to the reactor. The recovery step may becarried out using any suitable separating unit, for example, adistillation column. The recovered t-butyl acetate stream may alsocontain other reaction products and/or unreacted reactants including,for example, water and unreacted C₄ compounds.

[0028] Optionally, sec-butenyl acetate may be recovered from the productstream. The separated sec-butenyl acetate may be recycled to thereactor, or isolated for, for example, sale, direct use (eg as asolvent), or further processing. In one embodiment of the invention, thesec-butenyl acetate is thermally cracked back to butadiene and aceticacid. One or both of these starting materials may be recycled to thereactor, or sold as such.

[0029] As described above, t-butyl acetate, n-butenyl acetate andsec-butenyl acetate are recoverable from the product stream. This may beachieved by conventional methods, for example, by distillation. Incertain cases, however, the components of the product stream haverelatively similar boiling points. This may make conventionaldistillation difficult.

[0030] In a preferred embodiment, components of the product streamcomprising t-butyl acetate, n-butenyl acetate and sec-butenyl acetatemay be separated from said product stream by azeotropic separativemethods, for example using an azeotroping column. This techniqueinvolves, for example, introducing the product stream into anazeotroping colunm in the presence of water. Under these conditions, itis possible to form water/t-butyl acetate/n-butenyl acetate/sec-butenylazeotrope(s). The azeotrope may be recovered as a process stream,preferably, from an upper portion of the azeotroping column, morepreferably, as an overhead stream. When this process stream is allowedto settle, the organic components of the azeotrope, namely, t-butylacetate, n-butenyl acetate and sec-butenyl may form a layer separatefrom the water and may be decanted therefrom.

[0031] Any residual water may be removed from the decanted mixture bydrying the mixture further, for example, using molecular sieve treatmentor azeotropic drying. If desired, the azeotroping technique can beadapted by employing an organic azeotroping agent, for example,cyclohexane. Water separated in this manner may be recycled to theazeotroping column. The mixture of t-butyl acetate, n-butenyl acetateand sec-butenyl may then be conveniently separated by conventionaldistillation techniques. If desired, mixtures of t-butyl acetate andsec-butenyl acetate can be recycled to the reactor thus rendering theirseparation from one another unnecessary.

[0032] A stream comprising unreacted acetic acid and optionally, waterand/or reaction by-products may be recovered from the base of theazeotroping column. This stream may be recycled to the reactor. It maybe desirable to purify such streams prior to the recycling step,particularly, when tars are present. This may be achieved by introducingthe stream into a settling tank, and allowing the tars to separate fromthe remainder of the stream as a separate phase. This settling stepmaybe facilitated by the addition of water.

[0033] It should be understood that the azeotropic distillationprocedure described above may be applied to any process streamcomprising t-butyl acetate, n-butenyl acetate and sec-butenyl acetate.

[0034] The invention is illustrated by reference to the following Testswhich show how the composition of a C4 stream changes when subjected totreatment with acetic acid in an addition reactor, but without removaland recycle of the products. Hence the Tests illustrate the build-up ofby-products, some of which are valuable materials in their own right(e.g. di-isobutene) and some which are regarded of a lesser utilitywhich would be removed at intervals from a commercial process to Oreventundue build-up of involatile by-products.

TESTS 1 AND 2—REACTION OF ACETIC ACID WITH A CRUDE C4 STREAM

[0035] To 2699 g acetic acid and 28 g water was added decane (65 g—toact as an internal standard for subsequent Gas chromatographicanalysis), and butylated hydroxytoluene (2.4 g—polymerizationinhibitor). The mixture was charged to a 7.51 stainless steel Parrautoclave containing a cooling coil, and supplied with electric bandheaters and internal control thermocouple. The catalyst employed was aproprietary sulphonic acid ion exchange resin held captive inside astainless steel mesh bag fixed around the cooling coil. The action ofthe agitator enables liquid flow through the catalyst “bed”. The vesselwas sealed and pressure purged with nitrogen several times (to 10 barg),to remove air. The mixture was then heated with agitation at 1000 rpm to60 degrees C. (Test 1) nad repeated at 70° C. (Test 2). Once attemperature, some 886 g of crude C4 was added to the vessel undernitrogen overpressure. Excess nitrogen was used to increase pressure todesired 40 barg. This gave an acetic acid to butadiene ratio of 4.34:1.

[0036] The composition of the crude C4 stream is given in Table 1 below.

[0037] Reaction time zero was taken as the moment of C4 addition, andthe mixture was maintained at the stated temperature for a period of 4days while liquid samples were periodically withdrawn and analysed bygas chromatography. The results showed a kinetic profile of productmixture; tertiary-butyl acetate, secondary butenyl acetate and crotylacetate main products, with lesser amounts of tertiary butanol, C8olefins and acetates, C1 2 olefins, 4-vinylcyclohexane andoligomer-by-products, together with unreacted butadiene and acetic acid.

[0038] Table 2, below, shows the product composition (analysis by gaschromatography) from Test 1.

[0039] For Test 2 (carried out at 70° C.) using the same composition,results are given in Table 3, below.

[0040] It can be seen from these static Tests that tertiary butylacetate forms rapidly under the reaction conditions. It is thus apparentthat under suitable working conditions for treating C4 streams inaccordance with the present invention, the tertiary butyl acetate can berecycled to an addition reaction, optionally-together with otherproducts which may not be desired reaction products. It is also apparentthat under the static conditions of Tests 1 and 2 that, after reaching apeak, the concentration of tertiary butyl acetate gradually diminishes.It is believed that this is due to the formation of dimeric or otherproducts either from the tertiary butyl acetate itself or from theisobutene with which it is in equilibrium. It can also be seen from theTests that, in addition to such useful products as, for example,secondary butenyl acetate and crotyl acetate (which are easily separablefrom the starting materials), there is a gradual build-up of some otherproducts, e.g., C8 butadiene dimers, C8 acetates and C12 butadienetrimers which are also useful isolable products. TABLE 1 Composition ofCrude C4 stream obtained from Naptha cracker Component Percent (weight)Butadiene 63.2 Butene-1 11.1 Butene-2 6.9 C3's 0.1 C5's 0.015 Isobutane1.9 Isobutene 5.7 N-Butane 8.9 Total accounted for 97.815

[0041] TABLE 2 Product composition profile obtained at 60° C. Time C8 BDC8 C12 BD Sec- Crotyl (mins) dimers acetates trimers Oligomers t-BuOAcBuOAc acetate 4VCH t-BuOH 0 0.32 0.03 0.03 0.01 3.99 1.34 2.07 0.00 9.4129 0.55 0.05 0.07 0.01 113.14 5.25 3.35 0.06 44.04 64 1.25 0.27 0.230.02 209.38 15.84 9.19 0.06 74.01 93 2.16 0.43 0.45 0.03 208.85 25.3315.14 0.08 74.54 142 2.92 0.74 0.71 0.04 203.24 37.26 23.07 0.11 72.73202 3.12 1.20 1.14 0.10 193.07 50.94 32.51 0.17 69.93 269 3.70 1.65 1.550.23 180.62 64.80 43.12 0.20 67.37 322 4.17 2.11 1.99 0.45 169.08 75.8352.79 0.25 64.74 425 4.65 2.69 2.53 0.78 157.52 89.71 64.66 0.31 62.171369 7.31 5.21 5.67 3.98 102.14 155.93 140.23 0.84 48.82 1515 7.50 8.346.39 4.80 91.69 167.49 154.81 0.92 45.92 1645 8.07 8.86 6.68 5.23 87.95171.96 161.12 1.01 44.74 1740 8.28 6.56 6.82 5.40 85.32 174.08 164.921.03 43.96 2803 9.79 8.16 7.83 7.23 68.96 185.73 183.16 1.34 38.40 296610.19 11.81 8.14 7.66 64.96 186.57 187.96 1.40 37.03 3326 10.61 9.038.37 8.23 60.94 186.65 189.67 1.45 35.53 4292 11.48 9.74 8.74 9.44 53.40185.73 188.66 1.59 32.45 4464 11.79 10.07 8.94 9.82 50.97 183.80 188.661.62 31.56 4614 11.91 13.64 8.98 9.44 50.08 184.25 186.96 1.62 31.174759 12.08 13.63 8.94 9.26 49.27 184.86 186.59 1.65 30.89 5692 12.5914.33 9.28 10.11 43.77 176.20 182.88 1.65 28.43 5816 12.86 14.55 9.4010.50 42.56 176.04 180.45 1.68 27.90 5928 12.91 14.56 9.36 11.24 41.95175.12 169.92 1.68 27.65

[0042] TABLE 3 Product composition profile obtained at 70° C. Time C8 BDC8 C12 BD sec Crotyl (mins) dimers acetates trimers Oligomers t-BuOAcBuOAc acetate 4VCH t-BuOH 0 0.12 0.05 0.06 0.01 1.83 1.29 1.52 0.00 2.6412 0.39 0.10 0.05 0.10 122.36 4.36 3.04 0.08 45.02 25 0.91 0.06 0.140.10 195.98 11.70 6.78 0.14 58.69 36 1.24 0.08 0.28 0.10 203.69 18.2210.57 0.14 58.47 60 1.84 0.45 0.61 0.12 195.05 30.60 18.56 0.17 55.96117 3.10 1.15 1.36 0.23 173.51 55.88 36.28 0.22 52.14 149 3.66 1.61 1.770.38 161.98 68.70 46.49 0.25 49.75 176 4.56 2.49 2.79 1.06 141.45 92.4467.01 0.31 46.06 307 5.45 3.61 3.99 1.85 120.45 117.69 92.52 0.37 41.761312 8.93 6.94 7.57 5.75 70.92 170.84 158.47 0.62 29.67 1434 16.20 7.808.38 6.58 61.98 177.00 169.52 0.65 27.11 1711 10.96 8.20 8.85 7.60 56.36177.79 172.44 0.67 25.42 1775 10.60 8.44 8.76 7.48 54.84 179.11 173.480.70 24.89 2770 11.60 9.42 9.68 9.17 43.79 173.63 170.31 0.73 21.18 289211.92 9.77 9.66 9.55 41.06 172.13 168.51 0.76 20.16 3095 13.00 9.9410.55 9.89 39.34 170.17 169.60 0.73 19.63 3209 12.91 10.04 10.18 9.9138.33 169.21 168.99 0.76 19.21 4163 13.77 10.49 10.78 10.77 33.07 162.04160.75 0.76 17.30 4196 13.91 10.67 10.63 11.37 31.86 159.76 158.58 0.7616.76 4224 13.86 9.08 10.89 11.01 31.89 159.99 159.14 0.76 16.82

1. A process for treating a mixed C4 stream comprising iso-butene and1,3-butadiene, said process comprising: a) reacting acetic acid withsaid stream in an addition reactor, b) withdrawing from the additionreactor a product stream comprising iso-butene, sec-butenyl acetate,n-butenyl acetate and t-butyl acetate, and c) recovering n-butenylacetate from the product stream, characterised in that d) t-butylacetate is recycled to said addition reactor.
 2. A process as claimed inclaim 1 wherein isobutene is recovered from the product stream.
 3. Aprocess as claimed in claim 2 wherein the isobutene is recovered bydistillation.
 4. A process as claimed in claim 1 wherein isobutene isrecycled to the addition reactor.
 5. A process as claimed in claim 1wherein the mixed C₄ stream is obtained by the dehydrogenation of butaneor butene, or by the steam cracking of naphtha.
 6. A process as claimedin claim 1 wherein, in addition to the iso-butene and 1,3-butadiene, themixed C₄ stream comprises one or more of isobutane, n-butane, 1-butene,trans-2-butene, cis-2-butene, 1,2-butadiene, propadiene, methylacetylene, ethyl acetylene, dimethyl acetylene, vinyl acetylene,diacetylene, and C₅ acetylenes.
 7. A process as claimed in claim 1wherein, in the addition reactor, the relative mole ratios of butadienein the mixed C₄ stream to acetic acid is in the range 1:1 to 1:10.
 8. Aprocess as claimed in claim 1 wherein the addition reaction is carriedout at a temperature in the range 40 to 90° C.
 9. A process as claimedin claim 1 wherein the addition reaction is carried out using aheterogeneous catalyst.
 10. A process as claimed in claim 9 wherein theheterogeneous catalyst comprises a zeolite or a strong acid ion-exchangeresin.
 11. A process as claimed in claim 1 wherein water is present inthe addition reaction in an amount between 0.5 and 20 w/w %, based onthe total charge to the reactor.
 12. A process as claimed in claim 9wherein the addition reaction is carried out under conditions of highshear.
 13. A process as claimed in claim 1 wherein a polymerisationinhibitor is added to the addition reaction mixture.
 14. A process asclaimed in claim 1 wherein the n-butenyl acetate recovered from theproduct stream is hydrogenated to produce butyl acetate.
 15. A processas claimed in claim 1 wherein components of the product streamcomprising t-butyl acetate, n-butenyl acetate and sec-butenyl acetateare separated from said product stream by azeotropic separative methods.16. A process as claimed in claim 1 wherein di-isobutene is recoveredfrom the product stream.